Couple of comments/questions/suggestions...

1...What type of hunting do you intend to do? Stand or stalking? The reason I ask is because packing a heavy barrel rifle up and down the hills of someplace like Montana...we'll, you'll only do it once (speaking from experience)...hunting from a stand makes a 10+ pound rifle more tolerable...so consider your hunting style as you consider your purchase...

2...Caliber? Some calibers lend themselves more to long range hunting than others...600+ yards is a long ways off and very few people can actually accurately hunt at that range...and those that do, have spend a loooooot of time on the range and sling a lot of lead. There are a few people on this forum that can shoot out that far and do...RidgeRunner is one I'd talk to...

3...Do you want an off the shelf rifle or a custom build? I am betting not many average out of the box rifles will consistently shoot out past 600 yards without modifications, reloading and great (not good...great) glass. You might look at the Savage Model 10 FLCP/FCP series in 308...I have one I built to start long range shooting with and for an off the shelf rifle they are probably capable of the job...I am still breaking mine in and developing loads but and consistently hit an 8" plate at 250 yards after about 200 rounds through it.

Talk to some people who really long range shoot and not just those that play at it (like me)...at the end of the day, while the rifle is a critical element of long range shooting, as is the right bullet/load (handloading is almost a must!)...the shooter is more important and a lot of range time will be required...there's no substitute to range time. Enjoy the search and take your time!